AWR Logo

AUTOMATING A DOME

DOME ROTATION
We will discuss with you how to provide the right information so we can quote for a motor drive with sufficient power. We need to know the weight required to push it round, the shutter width and the degree of automation required.

The motor will be a large geared DC motor transferring power via a rubber wheel to the dome skirt on the inside. It must be capable of slewing about one degree per second. When the telescope is tracking an object near the zenith, the azimuth of the telescope changes rapidly, and so the dome has to rotate fast enough to maintain the telescope beam within the shutter aperture.

The wheel transferring the power must be pressing on the skirt quite hard in order not to slip so there should be another roller on the outside at this position to stop the dome moving off the track. This arrangement 'pinches' the skirt and so allows sufficient torque to be developed on the driven roller without slipping. A pneumatic tyre is preferred as it will take up any irregularities in the surface it is driving.

Let us work through an example.

A 4 metre diameter dome requires 10Kg force on the edge to start it turning. The force is measured with a spring balance (such as used by fishermen).
Now 10Kg force is practically 100 Newtons force. If we had a rubber drive wheel 0.2m diameter then we need 100 Newtons at 0.1 metre radius or 10Nm (Newton-metre).

The circumeference of the dome is pi*d = 12.5 metres. The circumference of the drive wheel is 0.625 metres. Therefore 20 revolutions of the small wheel will rotate the dome once. To move the dome at 1 degree per second is a small wheel speed of 3.33 RPM.

This is within the range of output power of motor / gearbox combinations costing about 130 pound sterling. The motor develops 15 watts running at 12V DC, 1.7 amps and produces 11Nm at 4 RPM. It would be advisable to have extra capacity in hand (by a factor of 2) so the rubber wheel in this case would be driven by a chain providng 2:1 reduction and it would travel at 0.5 deg per second.

AUTOMATING THE ROTATION
Those of you with Meade LX200 or AWR Intelligent Drive Systems are fortunate in that the azimuth of the telescope is available through the serial computer connection directly without any other equipment required. Other units that can connect to a computer that operate within a Planetarium programme or "The Astronomers Workbench" can get azimuth information provided by the computer. This information can then be used to automatically move the dome. The technique relies on knowing the azimuth of the dome shutter at any instant. By comparing the actual azimuth to the telescope azimuth it is a simple task to command the motor to move the dome to reduce the difference. A low cost method of finding the shutter position is to position magnets at equal distances around the moving part and a fixed sensor to detect. By using several sensors at a different spacing from the dome magnets it is possible to get greater angular resolution on the dome azimuth at the expense of more complex electronics. It is also possible by clever design to get an absolute position of the dome at one azimuth.

If the dome shutter allows for 10 degrees of movement in RA at the celestial equator (40 minutes of following) then the dome sensors must be separated by 10 degrees. When the azimuth detects it is 10 degrees since the last dome rotation, it must turn on the rotation motor and turn it off when the next sensor triggers. On slews, the dome must move x segments to cope with the azimuth change.

THE SHUTTER
Operation of the shutter is required at the start and end of the sessions only. For small domes AWR suggest it is not economic to provide this by motorizing the movement. A series of ropes and pulleys can go to a bicycle chain crank set-up which you can rotate by hand.

However we do have electronics designed for opening and closing a shutter and this operates a powerful DC motor. We use magnetic sensors for the 'open' and 'closed' positions and it requires a logic signal to start the motor moving. This was used on the NPL dome Shutter projects. The AWR ADVANCED INTELLIGENT DRIVE SYSTEM has relay outputs operated under handset control and this can be used to provide the correct signals for operation of the shutter.

AWR HOME PAGE

PRICING